Trunk piston engines are generally medium speed (300-1000 rpm), 4-stroke engines, in which a single lubricating oil is employed for lubrication of all areas of the engine, as opposed to the crosshead engines in which the crosshead allows for use of separate lubricants in the cylinder and in the crankcase. A trunk piston engine oil (TPEO) therefore has unique requirements for fuel compatibility, oxidative stability, viscosity increase control, and detergency.
Traditionally fuel oils used for the operation of trunk piston engines have ranged from heavy marine residual fuel to low sulfur distillate fuel. Recently, driven by health and environmental concerns, there has been increasing probability of future regulations mandating the use of low sulfur fuel for the operation of trunk piston engines. The use of low sulfur residual fuel requires that it is feasible for refineries to lower the sulfur level in residual fuel at a reasonable cost and effort. It is unknown whether there will be sufficient low-sulfur residual fuel oil available in the future, or whether low sulfur distillate fuel and gas oils will be used to a wider extent. It is therefore desirable to provide a trunk piston engine oil composition designed for use with low sulfur distillate fuel where the lubricating oil has a low base number but is capable of providing oxidative stability, viscosity increase control, and improved detergency performance.
Additives, especially metal-containing alkaline detergent additives, have been used for many years in TPEOs to neutralize acid combustion gases, maintain engine cleanliness, ensure compatibility of the lubricant with residual fuel oil, and control viscosity increase. However, it remains unclear if TPEOs formulated with additive technology developed for use with residual fuel oils will in fact be optimum for the low sulfur distillate marine fuels of the future due to differences in the characteristics of the fuels and differences in the environment of trunk piston engines due to the varying sources of fuels. For marine residual fuel operation, key performance parameters of trunk piston engine oils are almost exclusively driven by asphaltenes contamination. For distillate fuel operation, however, where the fuel contains no significant asphaltenes, key performance parameters are driven by combustion by-products from the distillate fuel. Therefore, the requirements for engines operated using low sulfur distillate fuels versus marine residual fuels are very different. As a result, this does not allow for performance read-across of a formulation from marine residual fuels operation to distillate fuel operation or vice versa.
The addition of dispersant, particularly in high concentrations, has traditionally been found to be detrimental to key performance parameters of Group I and/or Group II based trunk piston engine oils designed for use with marine residual fuel. It has been now been surprisingly discovered that Group I and/or Group II based marine trunk piston engine lubricating oil compositions designed for the lubrication of trunk piston engines operating on low sulfur distillate fuel, containing a detergent comprising at least one salt of an alkyl-substituted hydroxybenzoic acid wherein at least 90 mole % of the alkyl groups are C20 or greater, in combination with a succinimide dispersant derived from polyalkylene having a number average molecular weight (Mn) of 1400 to 3000, wherein the dispersant is present at greater than 1.2 wt. % on an actives basis and the lubricating oil composition has a Total Base Number of less than 30, leads to optimum performance in the areas of oxidative stability, viscosity increase control, and high temperature detergency.